In recent years, along with the spread to general users of portable terminals such as smartphones and cell phones, there has been proposed a system for improving convenience for a user's access, merchandise purchase, or the like and for improving services by providing information from a store to a user, providing point reduction or other various services, or the like, for example, by installing an ultrasonic beacon in a store to communicate with a user's portable terminal by using ultrasonic waves.
Now, considering technical and fundamental constraints that should be thought about when using a microphone of a portable terminal such as a smartphone or a cell phone, the following constraints (constraint 1) to (constraint 6) are found.
(Constraint 1) Frequency Characteristics of Microphone
Various models of portable terminals are present and new models are put on the market every day. Therefore, testing all models of portable terminals on a one-by-one basis leads to large cost and it is very important to avoid the large cost by using a program independent of the circumstances unique to each model of the portable terminal.
Particularly, in the case of a smartphone application, the field where the application is provided (Apple Store®, Google Play®, etc.) is not divided into the fields corresponding to the models of portable terminals, and therefore the smartphone application needs to be compatible with a plurality of models of portable terminals (including new models of portable terminals to be put on the market in the future) by using one kind of program.
In this regard, the microphone built in a portable terminal has frequency characteristics (frequency-dependent sensitivities) and especially the frequency characteristics in a high-pitched sound range (substantially 18 kHz or more) which is outside the audible range vary widely among portable terminals. Furthermore, it is difficult to measure the sound volume (which means a sound pressure level and the same applies hereinafter) in a state where the frequency characteristics of the microphone of the portable terminal are unknown.
Therefore, the variations in the frequency characteristics of the microphone present in each model of the portable terminal makes it difficult to measure the sound volume on the same standard by using one kind of program which can be used independently of the model of the portable terminal.
Specifically, even in the case of outputting sound having a certain frequency with a certain sound volume from a beacon, the measured sound volume vary according to the model of the portable terminal which measures the sound (due to a difference in the frequency characteristics of the built-in microphone).
Similarly, even in the case of outputting two sounds having pitches different from each other to some extent (by several hundred Hz) or more with the same sound volume, the volumes of these two sounds which can be measured by the portable terminal may be different from each other due to a difference in the frequency characteristics.
Thus, it is difficult to use a difference in the volumes of two sounds having pitches different from each other to some extent for a parameter of communication in a state where the frequency characteristics of the portable terminal are unknown.
(Constraint 2) Ambient Noise and Background Noise
There are various high-pitched noises around a store, such as noises from a rat repellent, a cockroach repellent, mosquito sound for preventing young people from hanging out, digital signage, and electrical appliances. Most of these noises occur with constant sound volume at a specific pitch (specific frequency).
It imposes a large cost burden to measure what kinds of noises are present around the store beforehand and the environment changes from moment to moment. Therefore, it is necessary to achieve a communication method less susceptible even if there is a noise in any kind of frequency band.
(Constraint 3) Doppler Effect
Since a user is likely to move even during receiving a sound signal, it is necessary to acquire a communication method in which the Doppler effect is considered.
Specifically, even in the case of outputting a sound having a certain pitch (frequency A) from a beacon, a frequency detected by the portable terminal is likely to deviate from the frequency A due to the Doppler effect. The deviation is determined according to the moving speed or moving direction of the user and therefore it is impossible to assume the deviation in advance.
According to the observation results of the inventors, change in frequency caused by the Doppler effect when a user uses a cell phone in a general using method is as much as plus or minus 200 Hz at maximum.
(Constraint 4) Available Bandwidth
The upper limit of a human audible range is said to be 18 kHz, 19 kHz, or 20 kHz though there are individual differences and differences by age. If a value is less than the upper limit, the sound would be heard by a human being (lower limit).
On the other hand, a typical sampling rate used when the portable terminal processes the audio file and audio input and output is 44,100 Hz, and the theoretical upper limit of a detectable frequency is 22,050 Hz (Nyquist frequency: one half of the sampling rate) (upper limit).
In fact, due to the characteristics of the microphone described in the constraint 1, filtering for keeping the quality (for example, a low-pass filter), or the like, there are many portable terminals whose detectable upper limit is around 21 kHz. Therefore, as a communication frequency between a beacon and a portable terminal, it is necessary to use a limited frequency band having a lower limit of 18 kHz, 19 kHz, or 20 kHz and an upper limit of 21 kHz or 22.05 kHz.
(Constraint 5) Available Modulation Scheme
There is a phase modulation as a scheme often used to convert CDMA or other digital data to an analog waveform signal.
The phase modulation is commonly used since it is resistant to ambient noise and its demodulation is not so difficult even if a signal level is low. If, however, a signal in the modulation scheme is output from a speaker, audible sound is output from the speaker during phase modulation and therefore it is difficult to achieve a sufficiently large sound volume, in other words, a sufficiently large arrival range of the signal, granting that even a frequency exceeding the audible range is used.
Moreover, the above easily occurs particularly in a piezoelectric speaker which is used for downsizing a beacon (because a film easily moves in response to a small change in voltage) and therefore it also hinders the downsizing of the beacon.
(Constraint 6) Fraud by Audio Recording
If a signal output from the beacon is detectable by the microphone of a smartphone or a cell phone, the signal can also be recorded. Therefore, when detecting in which store the portable terminal is present by using a beacon placed in the store as in this situation, it is possible to disguisedly act as if a signal output from the beacon in the store were detected by illegally recording and reproducing the signal output from the beacon in the store, which may lead to a serious problem depending on the form of a service to be used.
(Conventional Technology)
In Patent Document 1, there is proposed a presence detection method and system including: outputting signals having a plurality of frequencies with constant sound volume from a beacon in a store, assuming the combination of the frequencies to be unique to the store, receiving the signals having the plurality of frequencies from the beacon at a portable terminal, and identifying the store by determining which frequency is output.
The presence detection method and system in Patent Document 1 are strongly influenced by the frequency characteristics of the portable terminal (the aforementioned constraint 1). Specifically, if there is a portable terminal whose sensitivity to a specific frequency band is extremely low, the portable terminal cannot detect sound even if the sound is output in the frequency band. Moreover, the presence detection method and system are strongly influenced by the Doppler effect (the aforementioned constraint 3). Due to the Doppler effect, it cannot be accurately found which frequency is output while roaming with the terminal.
In addition, the presence detection method and system in Patent Document 1 are unresistant to ambient noise (the aforementioned constraint 2). The following problems are estimated: there is no method in which a signal output from the beacon is distinguished from ambient noise; practically the system is able to transmit only a few bits of information (assuming that the FFT resolution ranges from 10 to 100 Hz) due to the above (constraint 4) though a frequency band to be used has to be widened in order to increase the amount of information to be transmitted, and the like.
Moreover, the presence detection method and system in Patent Document 1 may also be configured to send a signal in a Morse code manner by turning on or off a specific frequency (referred to as “carrier”: a plurality of frequencies may be used) on the store side.
In this case, the content of a signal and the frequency used for the carrier are unique to the store and the transmission content is changed according to time. Therefore, it is considered that the amount of information which can be transmitted increases.
In the conventional example, however, if information is carried on a single carrier, the information is more likely to be demodulated as a different signal in the case where noise occurs in the frequency concerned and much information cannot be sent in a short time when using only a single carrier.
Moreover, determining whether ringing is made on the single carrier requires the setting of a threshold or the like. It is, however, extremely difficult to set an appropriate threshold value due to an influence of microphone characteristics (the aforementioned constraint 1).
Furthermore, it is necessary to change the threshold value for each portable terminal or for each location, thus requiring thousands of man-hours and complicating the program.
Still further, in the case of sending information on a plurality of carriers, each information is sent on each carrier and therefore synchronization between carriers cannot be achieved. Specifically, the number of ringing carriers at a certain moment is not constant and the timing of rising vary depends on each carrier. Accordingly, comparison for each frequency cannot be performed at detection and therefore it is necessary to observe the change with time of the carriers as in the case of the single carrier. In that case, the same problems as in the case of the single carrier occur.